web.xml

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The HTTP2 Upgrade Protocol

Table of Contents

Introduction

The HTTP Upgrade Protocol element represents an
Upgrade Protocol component that supports the HTTP/2 protocol.
An instance of this component must be associated with an existing
HTTP/1.1 Connector.

HTTP/2 connectors use non-blocking I/O, only utilising a container thread
from the thread pool when there is data to read and write. However, because
the Servlet API is fundamentally blocking, each HTTP/2 stream requires a
dedicated container thread for the duration of that stream.

Attributes

Common Attributes

All implementations of Upgrade Protocol support the
following attributes:

Attribute

Description

className

This must be org.apache.coyote.http2.Http2Protocol.

Standard Implementation

The HTTP/2 Upgrade Protocol implementation supports the
following attributes in addition to the common attributes listed above.

Attribute

Description

allowedTrailerHeaders

By default Tomcat will ignore all trailer headers when processing
HTTP/2 connections. For a header to be processed, it must be added to this
comma-separated list of header names.

compressibleMimeType

The value is a comma separated list of MIME types for which HTTP
compression may be used.
The default value is
text/html,text/xml,text/plain,text/css,text/javascript,application/javascript,application/json,application/xml
.

compression

The HTTP/2 protocol may use compression in an attempt to save server
bandwidth. The acceptable values for the parameter is "off" (disable
compression), "on" (allow compression, which causes text data to be
compressed), "force" (forces compression in all cases), or a numerical
integer value (which is equivalent to "on", but specifies the minimum
amount of data before the output is compressed). If the content-length is
not known and compression is set to "on" or more aggressive, the output
will also be compressed. If not specified, this attribute is set to
"off".

Note: There is a tradeoff between using compression (saving
your bandwidth) and using the sendfile feature (saving your CPU cycles).
If the connector supports the sendfile feature, e.g. the NIO2 connector,
using sendfile will take precedence over compression. The symptoms will
be that static files greater that 48 Kb will be sent uncompressed.
You can turn off sendfile by setting useSendfile attribute
of the protocol, as documented below, or change the sendfile usage
threshold in the configuration of the
DefaultServlet in the default
conf/web.xml or in the web.xml of your web
application.

compressionMinSize

If compression is set to "on" then this attribute
may be used to specify the minimum amount of data before the output is
compressed. If not specified, this attribute is defaults to "2048".

initialWindowSize

Controls the initial size of the flow control window for streams that
Tomcat advertises to clients. If not specified, the default value of
65535 is used.

keepAliveTimeout

The time, in milliseconds, that Tomcat will wait between HTTP/2 frames
when there is no active Stream before closing the connection. Negative
values will be treated as an infinite timeout. If not specified, a default
value of 20000 will be used.

maxConcurrentStreamExecution

The controls the maximum number of streams for any one connection that
can be allocated threads from the container thread pool. If more streams
are active than threads are available, those streams will have to wait
for a stream to become available. If not specified, the default value of
20 will be used.

maxConcurrentStreams

The controls the maximum number of active streams permitted for any one
connection. If a client attempts to open more active streams than this
limit, the stream will be reset with a STREAM_REFUSED error.
If not specified, the default value of 100 will be used.

maxHeaderCount

The maximum number of headers in a request that is allowed by the
container. A request that contains more headers than the specified limit
will be rejected. A value of less than 0 means no limit.
If not specified, a default of 100 is used.

maxHeaderSize

The maximum total size for all headers in a request that is allowed by
the container. Total size for a header is calculated as the uncompressed
size of the header name in bytes, plus the uncompressed size of the header
value in bytes plus an HTTP/2 overhead of 3 bytes per header. A request
that contains a set of headers that requires more than the specified limit
will be rejected. A value of less than 0 means no limit. If not specified,
a default of 8192 is used.

maxTrailerCount

The maximum number of trailer headers in a request that is allowed by
the container. A request that contains more trailer headers than the
specified limit will be rejected. A value of less than 0 means no limit.
If not specified, a default of 100 is used.

maxTrailerSize

The maximum total size for all trailer headers in a request that is
allowed by the container. Total size for a header is calculated as the
uncompressed size of the header name in bytes, plus the uncompressed size
of the header value in bytes plus an HTTP/2 overhead of 3 bytes per
header. A request that contains a set of trailer headers that requires
more than the specified limit will be rejected. A value of less than 0
means no limit. If not specified, a default of 8192 is used.

noCompressionStrongETag

This flag configures whether resources with a stong ETag will be
considered for compression. If true, resources with a strong
ETag will not be compressed. The default value is true.

This attribute is deprecated. It will be removed in Tomcat 10 onwards
where it will be hard-coded to true.

noCompressionUserAgents

The value is a regular expression (using java.util.regex)
matching the user-agent header of HTTP clients for which
compression should not be used,
because these clients, although they do advertise support for the
feature, have a broken implementation.
The default value is an empty String (regexp matching disabled).

overheadContinuationThreshold

The threshold below which the payload size of a non-final
CONTINUATION frame will trigger an increase in the overhead
count (see overheadCountFactor). The overhead count will
be increased by overheadContinuationThreshold/payloadSize so
that the smaller the CONTINUATION frame, the greater the
increase in the overhead count. A value of zero or less disables the
checking of non-final CONTINUATION frames. If not specified,
a default value of 1024 will be used.

overheadCountFactor

The factor to apply when counting overhead frames to determine if a
connection has too high an overhead and should be closed. The overhead
count starts at -10. The count is decreased for each
data frame sent or received and each headers frame received. The count is
increased by the overheadCountFactorfor each setting
received, priority frame received and ping received. If the overhead count
exceeds zero, the connection is closed. A value of less than
1 disables this protection. In normal usage a value of
3 or more will close the connection before any streams can
complete. If not specified, a default value of 1 will be
used.

overheadDataThreshold

The threshold below which the average payload size of the current and
previous non-final DATA frames will trigger an increase in
the overhead count (see overheadCountFactor). The
overhead count will be increased by
overheadDataThreshold/average so that the smaller the
average, the greater the increase in the overhead count. A value of zero
or less disables the checking of non-final DATA frames. If
not specified, a default value of 1024 will be used.

overheadWindowUpdateThreshold

The threshold below which the average size of current and previous
WINDOW_UPDATE frame will trigger an increase in the overhead
count (see overheadCountFactor). The overhead count will
be increased by overheadWindowUpdateThreshold/average so
that the smaller the average, the greater the increase in the overhead
count. A value of zero or less disables the checking of
WINDOW_UPDATE frames. If not specified, a default value of
1024 will be used.

readTimeout

The time, in milliseconds, that Tomcat will wait for additional data
when a partial HTTP/2 frame has been received. Negative values will be
treated as an infinite timeout. If not specified, a default value of
5000 will be used.

streamReadTimeout

The time, in milliseconds, that Tomcat will wait for additional data
frames to arrive for the stream when an application is performing a
blocking I/O read and additional data is required. Negative values will be
treated as an infinite timeout. If not specified, a default value of
20000 will be used.

streamWriteTimeout

The time, in milliseconds, that Tomcat will wait for additional window
update frames to arrive for the stream and/or conenction when an
application is performing a blocking I/O write and the stream and/or
connection flow control window is too small for the write to complete.
Negative values will be treated as an infinite timeout. If not specified,
a default value of 20000 will be used.

writeTimeout

The time, in milliseconds, that Tomcat will wait to write additional
data when an HTTP/2 frame has been partially written. Negative values will
be treated as an infinite timeout. If not specified, a default value of
5000 will be used.

The HTTP/2 upgrade protocol will also inherit the following limits from the
HTTP Connector it is nested with: